The Collins 75A-4 Thread.

Discussion in 'Amplitude Modulation' started by N6YW, May 16, 2017.

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  1. KA9JLM

    KA9JLM Ham Member QRZ Page

    What did not sound good about it ?
  2. N2DTS

    N2DTS Ham Member QRZ Page

    Both sounded a bit like cheap telephones (stock) and the KWS-1 was carrier and one sideband on AM which can sound ok but did not on the kws-1.

    Back then we were looking for bass that would rattle the equipment around on the desk, not sure why, it was just something to shoot for.
    I still enjoy a receiver where you can watch the woofer cone move in and out at 10 or 20 Hz...
    I like to almost zero beat with the BFO on and turn the volume up and VIBRATE the 12 inch woofer....
    The 75A4 would get down to about 300 Hz?
    The diode load on the R390 into the Marantz was where it was at.
  3. N2EY

    N2EY XML Subscriber QRZ Page

    For a long time, "Collins" was the top-of-the-line in Amateur Radio, at least to many many hams. Everything else was "almost as good".

    75A-4 was the last of the old-style heavy-iron Collins ham gear, and so had a certain mystique to it. A classic that marked the end of an era, because, as Collins moved to the S-line, then all the other rigmakers went to smaller/lighter.

    @K9STH probably knows how many of each model of 75A-x were made, and it would not surprise me if the 75A-4 numbers were the most.

    73 de Jim, N2EY
  4. KA9JLM

    KA9JLM Ham Member QRZ Page

    Yep, My twins are like that when I am running a phone patch.
    AF6LJ likes this.
  5. K9STH

    K9STH Ham Member QRZ Page


    As has already been mentioned, the 75A-4, and the 75A-3, have mechanical filters which greatly restrict the bandwith and that definitely affects how the receiver "sounds". Even when an 8 kHz mechanical filter is used, the low frequency "roll off" means that frequencies below around 300 Hz are eliminated.

    With the 75A-3, it is very easy to install a "filter" that consists of 2-resistors and 2-capacitors that is very wide. Then, the bandwidth selection becomes the crystal filter that does not have the steep skirts and, when in the widest position, wide enough for excellent AM reception.

    In addition, the frequency response of the audio amplifier section of the 75A-4 was designed for SSB and does not have the high audio frequency response that the other A-Line receivers have.

    As for the number of receivers manufactured:

    75A-1 = 2000

    75A-2 = 2200

    75A-3 = 1800

    75A-4 = 5840

    Somewhere around 200 of the 75A-4 were built at the Collins Radio plant in Toronto, Ontario, Canada.

    The serial number information, various differences in the 75A-4, and information on the earlier A-Line receivers can be found at:

    Glen, K9STH
  6. N2EY

    N2EY XML Subscriber QRZ Page

    Thanks Glen!

    So, the total number of 75A-1/2/3 comes out to 6000 - just 160 more than the number of 75A-4.

    Also, looking at the introduced/discontinued dates, it's clear that the 75A-4 was on the market much longer than any of its predecessor models.

    73 de Jim, N2EY
  7. K9STH

    K9STH Ham Member QRZ Page

    One thing about the 75A-4 is that there were a number of those receivers that were sold to the U.S. Military. The frequency ranges, for some of the bands, were changed to facilitate the military needs.

    Due, in part, of the friendship of Arthur Collins, Arthur Godfrey, and Lt. General Curtis Lemay, a number of 75A-4 receivers were purchased for use by the Strategic Air Command and some of those receivers were installed in B-36 bombers.

    Collins issued a document as an addition to the normal 75A-4 manual that details the changes made to the receivers that were delivered to SAC.

    Glen, K9STH
    N2EY likes this.
  8. VE3AJM

    VE3AJM Ham Member QRZ Page

    The low frequency distortion and the low end freq. roll off points were very noticeable in the stock receiver. Years ago, I swept the stock audio circuit and found the frequency response of approx. 180-2700hz overall. Not very good. The frequency response was quite restricted. So I had a closer look at the audio circuits. I found that for the high end, C147 and C148 510pfs around the noise limiter stage really limited the high end. So I clipped them out of the circuit. That change alone got the freq response out to approx. 8 kHz. An easy simple change. I noted the cap shunting the audio output transformer C103. A 0.01 uf cap. Changing that out to a 0.001 uf helped the high end out to approx. 10 kz.

    On the low end, C93 and C96, 0.01uf coupling caps around the noise limiter, were replaced with 0.1uf caps. C100 and C101 were also changed to 0.1uf. Just these simple changes bring the low end response down to 40hz. The noise limiter itself is problematic. Even with the limiter switched off, the tube still conducts some and clips the audio waveform. I live in a relatively noise free QTH so I simply bridged across it. No more clipping. I added a loop of negative feedback from the 6AQ5 plate back to the previous class A stage plate. A 1 meg ohm resistor. This helped smooth out the audio response with the stock tiny audio output transformer and all. Simple, easy component changes that make a huge difference to the audio.

    I plan to keep one 75A-4 in this configuration, and one with the Sherwood SE-3, bypassing entirely the inboard AM and product detectors and audio output stages, and using an outboard PP audio amplifier with a reasonable output transformer.

    I find the stock 75A-2 audio to be quite muffled. The bandwidth can't be opened wider than approx. 5kc, and theres the same tiny audio output transformer that Collins used. Although I do like the action of the crystal phasing control on that receiver. That may be the next receiver to look into in the shop.

    Al VE3AJM
  9. K9STH

    K9STH Ham Member QRZ Page


    Use of 510 pf capacitors as audio coupling capacitors is one of the primary reasons that the Heath TX-1 Apache has modulation problems. Replacing them with 0.01 mfd capacitors makes the difference between light and day! Then, unplugging the 6AL5 clipper tube rounds out the improvement.

    The bandwidth of the 75A-2 and 75A-3 can be increased by increasing the value of C-108, C-109, and C-110. C-108 and C-109 are 5 pf and C-110 is 10 pf. Going to something like 10 pf to 15 pf for C-108 and C-109 and something like 15 pf to 22 pf for C-110 should widen out the response considerably. Removing those capacitors is what is done to make the receiver even narrower.

    Of course, realignment of the i.f. circuits needs to be done after changing the capacitors.

    Glen, K9STH
    N2EY likes this.
  10. VE3AJM

    VE3AJM Ham Member QRZ Page

    I noted some audio riding along on the AVC line. Any ideas out there for filtering it out on the 75A-4.

    Al VE3AJM

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